The trend for modem mobile radios is to use multifunctional mobile communication appliances which have multimedia capabilities and, in addition to multiband operation (0.9/1.8-2/2.5 GHz), are also designed for multistandard operation (GSM/PCN/UMTS/WLAN). This requires the use of bandwidth-efficient linear modulation forms such as 8PSK (phase shift keying), QPSK (quadrature phase shift keying) or QAM (quadrature amplitude modulation). This results in particularly stringent linearity requirements for the transmission path in order to keep transmission errors in the case of output signals at high levels as small as possible. A power amplifier must therefore be arranged at the output of the transmission signal path, with linearity which is as good as possible over a wide range. Since power amplifiers in mobile radios represent a high proportion of the total power consumption, the power amplifier which has been mentioned should also consume little power.
High power amplifier efficiency, that is to say a high ratio of the RF power that is produced to the power that is required, is generally achieved in the maximum power area in which the RF transmission characteristic of the power amplifier has severe non-linearities, however. Good linearity of the power amplifier can be achieved only with low efficiency, that is to say with a low output power in comparison to the DC power required by the power amplifier. In the case of mobile radios that are known at the moment, it is impossible without considerable additional complexity to achieve good linearity and a low current consumption, that is to say a long battery operating life, at the same time.
In order to solve this problem, it has been proposed that the baseband signals be subjected to predistortion before being fed to the power amplifier. The baseband signals are in this case predistorted in such a way as to compensate for the non-linear output characteristic of the power amplifier, by means of the predistortion. The non-linearities in the transmission path are thus compensated for in a suitable form. This allows a high output power with the power amplifier consuming little power at the same time, without the non-linearities that result from this unacceptedly modifying the output signal.
Since the non-linearities in the transmission path depend on the amplitude of the baseband signal, but not on its phase, the amplitude of the baseband signal must be taken into account when determining the predistortion coefficients. The amplitude-dependent predistortion coefficients may, for example, be stored in a memory. An address calculation must be carried out on the basis of the baseband signal in order to select the correct predistortion coefficient from the memory.
Conventional methods for address calculation are magnitude addressing and square magnitude addressing. Magnitude addressing generally requires more complexity than square magnitude addressing, but magnitude addressing has the advantage of a uniform amplitude increase.
Furthermore, in the case of conventional transmitting devices, the predistortion for the components I and Q in the baseband signal must be carried out in real time, thus resulting in stringent requirements for the rate at which the computation operations are carried out. The computation power required in this case depends on the bandwidth of the signal, on the clock frequency, on the oversampling factor and on the number and complexity of the computation operations which are required in the predistorter. The required chip area and the power consumption of the predistorter increase with the computation complexity.
Transmitting devices with adaptive predistortion stages are described in the German Patent Applications with the file references 103 45 517.5 and 103 45 553.1. These applications are hereby included in the disclosure content of the present patent application.